Getting stem cells into the clinic is seen as the Holy Grail of medicine.

Injections of these cells, which have the special ability to transform into any other type of cell, have been paraded as the panacea for diseases, from Parkinson's to diabetes.

And recent advances, such as clinical trials where heart attack patients are to be injected with stem cells or research showing their possible ability to restore the sight of blind mice, suggest this hope could be becoming a reality.

But is this really the case?

Stem cell 'cowboys'

We are at the stage where some stem cell research is moving from animal models towards the clinic, says Professor Anne McLaren, a developmental biologist from Cambridge University.

But while there has been much progress, there has also been a lot of hype, she adds.

"And I believe stem cell research really mustn't be overhyped - it will be a good while even before the somatic (adult or foetal) stem cells are applied in the clinic, and a decade or more before embryonic stem cells are."

Skin has been a success story for stem cells

Some attempts to edge stem cells closer towards the clinic are faring better than others. In fact, stem cells have been used for years in bone marrow transplants for leukaemia, and using stem cells to grow skin to graft onto burns patients has also been a great success.

Presently, research looking at stem cells to repair eyes, cartilage and even the spinal cord is also looking promising.

But, according to Professor Richard Gardner, a stem cell expert based at Oxford University, people are "emphasising the promise" and "failing to highlight the problems that are yet to be overcome".

"And in the meantime," he says, "there is the real concern about the stem cell cowboys."

"You've got these people charging thousands of pounds to inject dubious cells into people suffering from diseases such as multiple sclerosis, and people who are chasing couples about to give birth and charging them thousands of pounds to store their baby's umbilical cord cells."

This is unethical, he says, because there are some fundamental problems regarding how these stem cells, whether they are adult, foetal or embryonic, will behave once they are in the body.

Too much, too soon

Dr Huseyin Mehmet, from the US-based Merck Research Laboratories, agrees.

I would be loathe to start sticking stem cells willy-nilly into patients

Huseyin Mehmet

"I do not doubt that stem cell transplantation will be a therapy of the future, but I think we ought to be focusing much more on basic mechanisms of stem cell differentiation - how stem cells make the decision to move from their stem cell state into a more committed phenotype.

"And I don't think we know anything near enough about how we can control what our stem cells do in the test tube, the animal and the patient.

"I think we are so far away from knowing that, that I would be loathe to start sticking stem cells willy-nilly into patients."

But while stem cell transplant advances may be further off than hoped, the field is racing ahead in other, perhaps more "blue skies", areas of research.

Dr Steven Pollard is a researcher at the Wellcome Trust Centre for Stem Cell Research, based at Cambridge University, and works on brain stem cells.

He says: "With brain stem cells, it would be very unrealistic to build up patients' hopes that these could be used as a repair mechanism in transplants, but what they will do is become a very useful tool to understand the basic biology of disease."

Unlimited access

By creating stem cells from an animal or human with a particular disease, such as motor neurone disease, scientists can watch what is going on in the cells as they change into other cell types, tracking any peculiarities in the process, seeing what happens as they develop and grow.

And understanding the mechanism of a disease can be key to providing insights into possible treatments, or even cures.

Dr Pollard also predicts stem cells will prove very useful for drug screening.

Stem cells may aid cancer research

"To have unlimited access to human neural material offers the opportunity to screen drugs to look at how we can affect cell behaviour, test toxicity and these sorts of things."

He adds that knowledge about the basic biology of stem cells is going to be important in cancer research.

"I think this will be one of the more likely success stories.

"For a lot of tumours, like brain tumours or breast cancers, it seems to be the case that there is a sub-population of cells within the tumour that you could call cancer stem cells. So understanding those and targeting those is a pretty major area."

'Unrealistic expectations'

So slowly but surely, stem cell research is progressing, and scientists agree it is indeed an exciting and promising area.

But it seems, initially, that the biggest advances may be in understanding how these special cells work, working out how they can be controlled and harnessed, and using them to explore our basic biology before they will find a permanent home in the clinic.

Professor Gardner says: "I'm very mindful of what happened 30 years ago when everyone in the field was promoting gene therapy - this area has limped along and there has really been no major pay off yet.

"I think already with the stem cell field there is a real concern about engendering unrealistic expectations among those who stand to benefit.

"It is important to highlight what may be possible, but at the same time highlight some of the formidable technical problems that one has to grapple."